It is known, as described in Japanese Patent Publication Nos. 9309/65, 4160/68, 14039/70, 29830/76, and 27253/80, U.S. Pat. Nos. 3,244,549 and 3,244,550 and British Pat. No. 1,135,540, that various phenol compounds can be used as electron-accepting compounds for the pressure-sensitive recording or heat-sensitive recording materials. These phenol compounds, however, are not completely satisfactory electron-accepting compounds, in that their color density when used in combination with electron donating colorless dyes is not as high as desired, and the stability of the color image formed is sometimes insufficient.
With respect to the increase in speed of heat-sensitive recording systems, extensive investigations have been made in recent years in order to improve color denisity, and several procedures have been proposed.
One proposed procedure is to control the melting point of the electron-accepting compound itself to within the range of from 60.degree. to 100.degree. C. However, it is difficult to control the melting points of phenolic compounds, which are now most widely used as electron-accepting compounds, within the above range. Furthermore, if the phenolic compounds are modified for that purpose, they become expensive. Thus, this procedure is not very suitable from a practical viewpoint.
Another method is disclosed in Japanese Patent Publication Nos. 17748/74 and 39567/76, in which an organic acid and a phenolic compound are used in combination with each other as the electron-accepting component, or a multivalent metal salt of a compound having an alcoholic hydroxyl group is used. In addition, Japanese Patent Publication No. 29945/76 discloses the use of a hydroxyethyl cellulose/maleic anhydride sald copolymer.
Japanese Patent Publication No. 27599/76 and Japanese Patent Application (OPI) No. 19231/73 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") disclose the addition of waxes.
Japanese Patant Application (OPI) Nos. 34842/74, 115554/74, 149353/75, 106746/77, 5636/78, 11036/78, 48751/78 and 72996/81 disclose that nitrogen-containing organic compounds such as thioacetanilide, phthalonitrile, acetamide, di-.beta.-naphthyl-p-phenylenediamine, fatty acid amide, acetoacetic acid anilide, diphenylamine, benzamide, and carbazole, or heat-fusible materials such as 2,3-di-m-tolybutane, and 4,4'-dimethylbiphenyl, or carboxylic acid esters such as dimethyl isophthalate, diphenyl phthalate, and dimethyl terephthalate can be added as sensitizers. In addition, Published British Patent Specification No. 2,074,335A discloses the addition of hindered phenols.
However, heat-sensitive recording materials produced by any of the above methods are still not fully satisfactory in color density and the color-forming property.
Several methods have also been proposed with respect to increasing the stability of the color image in heat-sensitive recording materials. Japanese Patent Publication No. 43386/76 discloses a method in which phenol derivatives such as 2,2'-methylenebis(4-methyl-6-tert-butylphenol) are added. Japanese Patent Application (OPI) No. 17347/78 discloses the addition of water-insoluble modified phenol resins, such as resins modified with rosin. Japanese Patent Application (OPI) No. 72996/81 discloses the addition of terephthalic acid esters. In addition, Published British Patent Specification No. 2,074,335A discloses the incorporation of hindered phenols.
However, heat-sensitive recording materials produced by the above methods are still not fully satisfactory with respect to the stability of the image.